1. Technical Field of the Invention
The present invention relates to an electron beam lithography system and a method for applying a narrowly converged electron beam to a specimen substrate such as a semiconductor substrate to write a pattern thereon, and more particularly to an apparatus and a method for deflecting an electron beam in an electron beam lithography system.
2. Description of the Related Art
In an electron beam lithography system, a narrowly converged electron beam is scanned and applied sequentially to target positions on a specimen substrate to form a pattern thereon. As shown in FIG. 1 of the accompanying drawings, an electron beam lithography system comprises an electron optical column 1, a specimen chamber 6, and a control system 9. The electron optical column 1 has an electron gun 2, at least one of electromagnetic lenses 3 and 4, and at least two deflectors 5. Each of the two deflectors 5 deflects an electron beam in the X and Y directions, and comprises deflection plates, a deflection amplifier, and a deflection power supply, and the like. The electron optical column 1 has a function for converging an electron beam emitted from the electron gun 2 to be smaller by the electromagnetic lenses 3 and 4, deflecting the electron beam by the deflectors 5 which apply electromagnetic forces, and applying the electron beam to a substrate 8 at a desired position on its surface. Further, the electron optical column 1 has a blanking function for blocking the electron beam in response to an external signal. The electron beam lithography system is capable of writing graphic patterns on a substrate by the focusing, deflecting, and blanking functions of the electron beam in the electron optical column 1. In the specimen chamber 6, there are provided a specimen stage 7 for holding the substrate 8 and moving the substrate 8, and a measuring mechanism (not shown) for measuring the position of the specimen stage 7. The control system 9 has a function for controlling the overall apparatus including the electron optical column 1, the specimen stage 7, and the like by a computer, and controlling a deflection amount of the electron beam and the position of the substrate in a pattern writing process.
The electron beam lithography process is such a process that a desired graphic pattern is written by deflecting the electron beam 10 with the deflectors 5 to apply an appropriate amount of electric charges to only necessary portions of an electron beam resist which has been coated on the surface of the substrate 8, as shown in FIG. 2. Applying an electron beam to an electron beam resist and exposing the electron beam resist is referred to as electron beam exposure, and a rectangular region swept by an electron beam is referred to as a field.
Processes of deflecting an electron beam include a raster-scan process and a vector-scan process. According to the raster-scan process, one field is sequentially and fully scanned from one end to the other at the same speed, and the electron beam is applied to only an area where a graphic pattern is to be written and is blocked for an area where no graphic pattern is to be written by a blanking mechanism. This process is disadvantageous in that it wastes a long period of time because the electron beam is deflected in the area where no graphic pattern is to be written.
According to the vector-scan process, the electron beam is not applied to, but skips, an area where no graphic pattern is to be written, and the electron beam is deflected only in an area where a graphic pattern is to be written. While this process is advantageous in that the electron beam is not applied to, but skips, the area where no graphic pattern is to be written, it requires a period of time limited by the frequency band of the deflection amplifier to move the electron beam from a position where writing of one graphic pattern finishes to a position where writing of a next graphic pattern starts. Therefore, if one field contains many small graphic patterns, then a waiting time required for the electron beam to move from a graphic pattern to another graphic pattern increases.
Noise of an electronic circuit can be represented in the following:
N=Kxc3x97V maxxc3x97f1/2xe2x80x83xe2x80x83(1)
Where,
N . . . noise voltage,
Vmax . . . maximum value of the operating voltage of the electronic circuit,
f . . . frequency band, and
K . . . proportionality constant.
In the raster-scan process, the frequency band can be limited because the deflection direction, the deflection amount, and the deflection speed of the electron beam are constant. Therefore, the noise of the deflection amplifier can be reduced. In the vector-scan process, however, the deflection direction, the deflection amount, and the deflection speed of the electron beam are not constant depending on the graphic pattern to be written. After one graphic pattern has been written, it is necessary to move the electron beam quickly to a position where writing of a next graphic pattern starts. In order to lower the noise of the deflection amplifier to a certain value or less, the frequency band needs to be limited to a low frequency range, and there is a limit to acceleration of travel time of the electron beam.
In a conventional electron beam lithography system based on the vector-scan principle, when a solid graphic pattern is written, as shown in FIG. 3, the electron beam is scanned by a given width in the X direction, and thereafter deflected slightly in the Y direction and then scanned by the given width in the X direction. That is, the electron beam is deflected several times to fully scan a desired field in the X and Y directions.
FIGS. 4A and 4B are graphs showing deflection signals in the X and Y directions for drawing the rectangular graphic pattern shown in FIG. 3. FIG. 4A shows an X deflection signal and FIG. 4B shows a Y deflection signal. In FIGS. 4A and 4B, the horizontal axis represents time and the vertical axis represents the deflection signal (voltage signal). In this process, because the frequency band of the deflection signal (voltage signal) is limited to a low frequency range, the deflection speed of the electron beam is low, thereby requiring a long period of time to write the graphic pattern.
In order to solve the above problem, there has been proposed a method in which the deflection signal is divided into a main signal (voltage signal) having a low frequency and a large amplitude, and an auxiliary signal (voltage signal) having a high frequency and a small amplitude, whereby the electron beam is deflected in a large area by the main signal, and in a small area by the auxiliary signal. Because the noise of the signal is expressed by the equation (1), the overall noise can be reduced by dividing the deflection signal in this manner. According to a specific process, as shown in a conceptual view of FIG. 5, there has been proposed a method in which deflection plates dedicated for the main signal and the auxiliary signal are used. In the process shown in FIG. 5, main deflection plates 5-1, 5-2 and auxiliary deflection plates 5-3, 5-4 are provided. A main signal generator 11 and a main signal amplifier 13 are connected to the main deflection plates 5-1, 5-2, and an auxiliary signal generator 12 and an auxiliary signal amplifier 14 are connected to the auxiliary deflection plates 5-3, 5-4. In FIG. 5, only the main deflection plates 5-1, 5-2 and the auxiliary deflection plates 5-3, 5-4 for use in one direction, e.g., the X direction (or the Y direction) are illustrated. This method is disclosed in Japanese patent publication No. 1-52894, Japanese laid-open patent publications Nos. 57-90858, 11-224636, and 6-19639, for example. However, the method shown in FIG. 5 is problematic in that because the number of deflection plates increases, the spatial efficiency of the electron optical system is lowered.
According to another method, as shown in a conceptual view of FIG. 6, it has been proposed to connect the circuit""s common of auxiliary signal generators and auxiliary signal amplifiers to the output of a main signal for thereby adding main and auxiliary signals. Specifically, in the method shown in FIG. 6, auxiliary signal generators 12 and auxiliary signal amplifiers 14 are connected to main deflection plates 5-1, 5-2, and the auxiliary signal generators 12 are connected to a main signal generator 11 and a main signal amplifier 13. In the method shown in FIG. 6, however, because not only the circuit""s common of the power supply for the auxiliary signal generator 12, but also the circuit""s common of the power supply for the auxiliary signals (not shown) and of the auxiliary signal amplifier 14, and also the circuit""s common of the input and output circuits, connected to the auxiliary signal generator 12 must be kept in the same potentials of the circuit""s common of the auxiliary signal generator 12, many circuits are required to be newly provided separately from the main signal generator 11, resulting in a complex circuit arrangement. This method is disclosed in Japanese laid-open patent publication No. 8-45460, for example.
It may be possible to connect the output of a main signal and the output of an auxiliary signal to each other. However, this method is not practically feasible because the auxiliary signal is governed by the main signal and cannot exercise its function at all. This method may be explained by analogy with a painting process where when a picture is drawn on a large canvas, details are drawn by a large paintbrush carried by both a person and a large crane, with the person being useless.
The method illustrated in FIG. 6 may be explained by analogy with a painting process where a person is suspended by a large crane and the suspended person draws details of a picture with a small paintbrush. Though the circuit of the process is complex, the process can be reduced to practice.
In the vector-scan process, the amount of charges applied to a point on a graphic pattern can be controlled by changing the time in which the electron beam stays on the point. Exposing all resist molecules in the irradiated area to the electron beam is referred to as full exposure, and exposing part of resist molecules in the irradiated area to the electron beam is referred to as intermediate exposure. When the resist in full exposure is developed, the resist is dissolved away. When the resist in intermediate exposure is developed, the resist which is not exposed remains undissolved. Continuously changing the amount of exposure is referred to as gradient exposure. When the resist in gradient exposure is developed, the amount of resist which remains undissolved corresponds to the amount of exposure. The gradient exposure makes it possible to process a resist according to a three-dimensional pattern.
According to the conventional gradient exposure, it has been customary to change the deflection speed of the electron beam for changing the amount of exposure. In order to change the deflection speed of the electron beam, it is necessary to specify parameters determining the deflection speed each time the deflection speed is to be changed, thus wasting a long period of time.
The present invention has been made in view of the above drawbacks. It is therefore an object of the present invention to provide an electron beam lithography system and a method which can write a pattern having a desired width at a high speed while maintaining positional accuracy of the written pattern.
Another object of the present invention is to provide a writing method which can eliminate a dead time in gradient exposure carried out by an electron beam lithography system.
The present invention relates to an electron beam deflection apparatus in which a deflection signal is divided into a main signal having a low frequency and a large amplitude and an auxiliary signal having a high frequency and a small amplitude for applying them to deflection plates, and the main signal is applied directly to the deflection plate and the auxiliary signal is applied to deflection plates through a capacitive coupling.
In this apparatus, the frequency band of the main signal (voltage signal) is limited to a low frequency range for reducing noise even at a large amplitude, and the voltage range of the auxiliary signal (voltage signal) is reduced for reducing noise even in a high frequency range.
According to the present invention, in the case of writing a straight line having a certain width or a solid graphic pattern, the electron beam is deflected at a high speed in a certain direction for a certain width using the auxiliary signal in synchronism with the deflection of the electron beam by the main signal. Therefore, it is possible to write a line which is as thick as many times the diameter of the electron beam. Consequently, a graphic pattern having a certain width can be written at a high speed.
By using an auxiliary signal having a nonlinear waveform which is asymmetric with respect to the zero level, the distribution of the amount of applied charges can be given a gradient in the direction in which the electron beam is deflected by the auxiliary signal.